To do this, I read:
https://freefilesync.org/manual.php?topic=external-applications
In the "Customize context menu" section of the "Options" dialog box of FreeFileSync 13.5, I then created the following custom context menu:
kompare -c "%local_path%" "%local_path2%"
/bin/sh: line 1: kompare: command not found
I tried using other commands, but FreeFileSync issues the same error message. I also tried specifying the full path to the command's executable, but the error persisted.
In Linux Mint (which is based on Ubuntu & Debian), how does one successfully create a custom context menuitem in FreeFileSync?